The cosmos 2 has plenty of room for a rad. Good choice btw, very nice case

Here is a question though: why would you rather have a slim rad with 6 fans than a fat rad with 3 fans? A fat rad is basically a quiet rad.

I would much rather have a fat rad with 3 good push fans (high static pressure) than a rad with lower heat transfer surface area and 6 fans for a couple of practical reasons.

1: Good fans cost money! If you want a fan that has performance and silence, it costs ~ 20 bucks (120mm). So 6 fans vs 3 fans will cost you ~ 60 bucks.

2: A fat rad will allow you to expand up to running a cpu + gpu x 2 + mobo block while keeping your fans silent. Putting the same components on a slim rad will call for higher fan noise.

However, for just cooling a single component, a 360 slim rad is still technically overkill and thus can be run in silence with 3-6 good fans.

Now to answer your fitting questions. All watercooling parts use g 1/4 threads (some rare rare parts will use g 3/4). So your fitting has a threaded part and then the barb. The size of the barb is INDEPENDENT of the threaded side size.

So basically do what looks cool! Many people love the fat 1/2 '' ID and it probably will shave a degree or three off your temps, but 3/8 '' ID can be easier to rout as it's not so fat, and you won't run into problems where you can't fit two MASSIVE fittings on one small block, although many blocks can handle 2x 1/2'' with no problems.

Oh yeah, and only 7/16'' inner diameter with 5/8'' outer diameter will work with the tubing you bought. It is a good size as 3/8 > 7/16 > 1/2.